The present invention relates generally to a medical devices, and, more particularly, to implantable medical devices.
Congestive heart failure is a serious condition affecting at least five million Americans. Patients diagnosed with heart failure have an extremely poor long-term prognosis. The average life expectancy of a person suffering from chronic heart failure is approximately five years. Because of the severity of chronic heart failure, a need exists for developing efficacious therapies for this disease. The patient's clinical and hemodynamic status determines the baroreflex sensitivity (BRS), which is a measure of the ability of that individual's heart to react to changes in blood pressure by changing heart rate. The BRS of a chronic heart failure patient parallels that patient's clinical and hemodynamic status. Thus, BRS provides an indicator for the efficacy of a drug therapy or a ventricular resynchronization therapy. BRS may also be used to predict arrhythmic events and mortality in these patients.
In recent years, autonomic markers such as heart rate variability (HRV) and BRS have been recognized as a good indicator of a deteriorating heart. While HRV deals with the changes in the RR interval as a single variable, BRS measures the relationship between input and output signals in a feedback system. Typically, BRS takes systolic blood pressure as an input and RR interval as an output. Several methods have been suggested to measure BRS such as bolus injection of vasoactive drugs (e.g., phenylephrine), the Valsalva maneuver, and mechanical alteration of transmural carotid sinus pressure by means of the neck chamber. Such past techniques merely provide a snapshot in time of BRS and the results of such BRS tests must be compared against test results of other patients in order to determine whether the BRS indicate a worsening heart failure condition.